Mitigating Global Warming Through Earth Science: Geological Strategies

Global warming poses a significant challenge for our planet, characterized by a prolonged rise in Earth’s average surface temperature primarily linked to human activities (Figure 1). These activities involve the emission of greenhouse gases like carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) into the atmosphere over extensive periods. These gases capture heat within the atmosphere, resulting in what is known as the greenhouse effect. The impacts of global warming are profound and far-reaching. Increasing temperatures have resulted in intense and frequent weather events. For instance, the destructive bushfires in Australia between 2019 and 2020 devastated millions of acres, destroying thousands of homes and causing the deaths or displacement of close to 3 billion animals (Fletcher et al., 2021).  Another case is the increasing intensity of hurricanes in the Atlantic Ocean, exemplified by Hurricane Maria in 2017, which caused widespread devastation in Puerto Rico, resulting in significant loss of life and infrastructure damage. The melting of ice caps in the polar regions contributes to the increase in sea levels, endangering coastal communities. The swift decline of Greenland’s ice sheet, shedding 532 billion tons of ice in just 2019, serves as a stark illustration of this phenomenon.

India, with its vast and diverse geography, is particularly vulnerable to the effects of global warming. The country experiences a wide range of climate-related challenges, from intense heatwaves in the northern plains to devastating floods in the north-eastern states. For instance, the 2015 heatwave in India claimed over 2,500 lives, highlighting the severe impact of rising temperatures (Pattanaik et al., 2017). Rising temperatures are also exacerbating water scarcity in already drought-prone regions, affecting agriculture and livelihoods. The 2019 drought in Maharashtra, which led to significant crop failures and water shortages, is a stark example. The monsoon, which is crucial for India’s agriculture, is becoming increasingly erratic, leading to both droughts and floods. In 2018, Kerala experienced unprecedented flooding during the monsoon season, resulting in over 400 deaths and massive displacement. Coastal areas are facing the dual threat of rising sea levels and increased frequency of cyclones, endangering millions of people. Cyclone Amphan in 2020 caused extensive damage in West Bengal and Odisha, displacing thousands and resulting in significant economic losses. Additionally, the Himalayan glaciers, which are a vital water source for many rivers in northern India, are retreating at an alarming rate. The Gangotri Glacier, which feeds the Ganges River, has been retreating rapidly, threatening water supply for millions. These examples illustrate the urgent need for India to address the impacts of global warming to safeguard its people and environment.

In response to these challenges, India has laid out a comprehensive plan to mitigate global warming. The nation is dedicated to lowering the intensity of its greenhouse gas emissions and boosting the proportion of renewable energy in its energy portfolio. As part of its efforts to combat climate change, India has established a goal to achieve carbon neutrality by 2070 (Bhattacharyya et al., 2022). This long-term goal reflects the country’s dedication to significantly lowering its carbon footprint and transitioning towards a sustainable future. To achieve these ambitious targets, India is exploring various strategies, including underground geological sequestration. This encompasses the capture of CO₂ emissions from industrial sources and their injection into deep geological formations, such as exhausted oil and gas fields, non-mineable coal seams, tight sandstones, and saline aquifers, for storage. By injecting CO₂ into these underground reservoirs, India can effectively sequester large quantities of CO₂, preventing it from entering the atmosphere and contributing to global warming.

One of the most promising types of geological formations for CO₂ sequestration is shale rock (Hazra et al., 2022). Shale formations are abundant and widely distributed around the world, making them attractive targets for CO₂ storage. These rocks are characterized by their fine-grained texture and low permeability, which makes them capable of trapping CO₂ effectively. The mechanism of CO₂ storage in shale formations involves several processes, including structural trapping, solubility trapping, and mineral trapping. Structural trapping occurs when CO₂ is injected into a reservoir rock positioned below an impermeable cap rock layer, which prevents the gas from migrating upwards. Solubility trapping involves the dissolution of CO₂ into the pore water within the shale rock. Over time, the dissolved CO₂ can react with minerals in the rock to form stable carbonate minerals, a process known as mineral trapping. This ensures that the CO₂ is permanently sequestered and cannot escape back into the atmosphere.

Adsorption is a crucial mechanism for storing CO₂ in shale formations (Hazra et al., 2022). It is a process by which CO₂ molecules adhere to the surfaces of shale rocks through weak chemical bonds. Shale rocks, which contain significant amounts of organic matter and clay minerals components have high surface areas, which provide a lot of space for CO₂ molecules to adhere to. Upon injection into a shale formation, CO₂ molecules interact with the surfaces of the rock. These molecules are then attracted to and held on these surfaces through adsorption. This is similar to how a sponge soaks up water, but on a molecular level. Because the surfaces of the shale are so extensive, they can hold a large number of CO₂ molecules, significantly enhancing the storage capacity of the formation.

The efficiency of adsorption in shale formations depends on several factors. First, the composition of the shale plays a critical role. Shale rich in organic matter and certain types of clay minerals, like kaolinite, and illite is particularly effective at adsorbing CO₂. The organic matter in the shale has a high affinity for CO₂, meaning it can attract and hold more CO₂ molecules. Second, shale properties, such as porosity and permeability, are important. Higher porosity means more surface area is available for adsorption. However, shale typically has low permeability, which helps trap the CO₂ in place once it is adsorbed.

Pressure and temperature conditions also significantly impact adsorption capacity. Higher pressure enhances adsorption, while lower temperature diminishes it. This is because higher pressure forces more CO₂ molecules into the rock, while lower temperatures slow down the movement of molecules, making them more likely to stick to the surfaces. Therefore, deep underground formations, where pressures are high and temperatures are relatively stable, are ideal for CO₂ sequestration. Additionally, the presence of other gases, like CH₄ or N₂, can influence CO₂ adsorption. For example, CO₂ may compete with methane for adsorption sites in the shale. In some cases, injecting CO₂ can even displace methane, which can then be captured and used as an energy source, a process known as enhanced gas recovery (Hazra et al., 2022).

The potential of shale formations for CO₂ sequestration is immense. According to estimates, the global capacity for CO₂ storage in shale formations could be in the range of hundreds to thousands of giga tonnes. This capacity, combined with the widespread availability of shale formations, makes them a promising option for large-scale CO₂ sequestration. In addition to shale formations, CO₂ can be sequestered in other geological formations such as coal beds, tight gas formations, and salt formations. Each of these has unique characteristics that make them suitable for CO₂ storage. Coal beds and tight gas formations are effective for CO₂ sequestration due to their unique properties. In coal beds, CO₂ is adsorbed onto coal particles during coal bed methane (CBM) recovery, displacing methane that can be used as clean energy while securely storing CO₂. Tight gas formations, with their low permeability, trap CO₂ via structural confinement and adsorption, enhancing natural gas recovery. Salt formations, like salt domes, provide secure storage with their impermeable properties, containing CO₂ in cavities and porous regions deep underground. These geological options offer varied approaches to long-term CO₂ storage, contributing to sustainable climate solutions.

An example of field implementation of CO₂ sequestration is the Sleipner field in the North Sea, Norway. Since 1996, the Sleipner project has injected CO₂ separated from natural gas into a saline aquifer situated 1,000 meters beneath the seabed (Boait et al., 2012). This initiative has effectively prevented around one million tons of CO₂ emissions each year, showcasing the practicality and efficiency of storing CO₂ in geological formations. Another notable example is the SACROC unit located in the Permian Basin of western Texas, USA. Geologically, the SACROC Unit comprises extensive deposits of limestone and thin shale beds (Vest, 1970). CO₂ injection has been employed at this site for over 35 years to enhance oil recovery. According to research by Han et al. (2010), analysis of production and injection data indicated that approximately 93 million metric tons of CO₂ were injected into the subsurface from 1972 to 2005. India is still in the early stages of implementing large-scale CO₂ sequestration projects. However, the country possesses significant potential for such initiatives, particularly given its diverse geological formations suitable for CO₂ storage.

Earth science, through innovative approaches like CO₂ sequestration in geological formations such as shale, coal beds, tight gas formations, and salt formations, offers promising solutions to mitigate global warming. These techniques leverage the natural storage capabilities of the Earth’s subsurface to securely trap CO₂ emissions, preventing their release into the atmosphere. By utilizing geological knowledge and advanced technologies, such as enhanced gas recovery and adsorption mechanisms, greenhouse gases emissions will be significantly reduced and advance towards a sustainable future. This integrated approach underscores the pivotal role of earth sciences in addressing climate change challenges on a global scale. Table 1 shows the CO2 sequestration potential of different geological formations:

Table 1. Comparative table showing the CO₂ sequestration potential of various geological formation

References

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